There are a few key things I've noticed in the work, marketing and attitudes of the successful artists I follow, which I think hold true in most instances.
Skill
You have to be skilled at what you do. That doesn't necessarily mean skilled in every aspect of it, but in the aspect most vital to its success. Think of humour comics. Not all of those artists are skilled artists. But what they are skilled in is comedy. Beat, pacing, juxtaposition, relatability, and twisting the absurd - all of those things are more important than the art for such a comic strip. (Though nice art certainly helps!)
If you want to write story-driven comics, you need skill in both art and how to write a compelling story. Pace, tone, characterisation. Avoiding stale clichés, writing convincing and emotive dialogue, creating an interesting narrative - all of these are vital skills if you want to attract and maintain a readership. And, of course, writing a story nobody else is telling. Joining a writers group - one which gives honest critique - is a good idea here.
As for art, you need solid fundamental skills. A lot of artists with simple visual styles nevertheless have impeccable design language skills underpinning that style - shape, balance, colour theory and composition being the strongest. Artists with more detailed illustrative styles will also have these, along with an eye for form, lighting, volume and depth, among others. Drawing from life is the best possible advice I can give anyone, regardless of how simple or complex they intend for their style to be. You become skilled by training that skill, so - train.
Marketing
The big thing with marketing, and the thing which will save you from wasting years of your life chasing the coat-tails of trends set by other artists, is to know what it is you are offering to people in your work. Why do they want to look at your work? Why do they want to read your story? What do you aim to give them? An adventure? A calming escape from the doldrums of reality? A belly-laugh? All of the above? For me, I know what I am giving my audience with my work overall, as well as with each page. I know who my stories will appeal to. As such, I know which market which wants what I have to give. (It's middle-grade to YA fantasy/adventure graphic novels. I may be new to it, but it's been a duck-to-water process so far. I picked the right field.)
The other thing with marketing - and this is a big one, which is hard to achieve - is that you do not want to chase trends! I know 'find your own voice' is very ephemeral advice, but I'm not suggesting you launch a search party for it. Rather, spend time exploring. It may take years, but in time, you'll begin to form a clearer picture of who you are as an artist, and what you have to offer. Chasing trends may pay the bills, but you will likely burn out, and you're not likely to achieve the acclaim of those who set the trends in the first place.
Attitude
You know how, when you're desperately looking for something, and you can't find it anywhere, but when you give up looking that thing seems to pop back out of the void and fall into your lap? Yeah, art is like that. If you fixate on a rush to success, especially to the point where your natural creativity and drive to explore become stifled, you're much less likely to find a path that's both creatively satisfying and marketable. Like Lyra and her alethiometer, you have to still your mind. Drift through the creative process. Don't blind yourself to your goals, but allow them to float to the side. They're less important than your creativity.
Of course, you have to pay your bills too. I'm entering into professional illustration rather late - I already have a degree in teaching, so I teach two days a week. That covers rent, bills and food. I want my art and writing to start making a liveable wage, but I've accepted that point may be years away, and there is no reliable way to rush the process. Whatever you do to make ends meet is valid, be it commission work, a part-time job, or what have you. Just make sure it leaves you with enough energy to create.